The transmission of digital signals derived from a plurality of signal sources by multiplexing them in the time division arrangement is well known. In the well-known time division multiplexing systems, however, the digital signals of a plurality of channels to be subjected to the time division multiplexing generally must have either the same bit rate or bit rates which are an integral multiple of one another. Otherwise, respective channel signals have to be synchronized by using a technique such as a start-stop system. Accordingly, in the transmission system that employ heretofore-known time division multiplexing techniques, the kinds of signals to be multiplexed are limited, and circuits and apparatus to be incorporated in a particular system must be designed exclusively for use in the system and thus cannot be applied to other systems having different clock rates.
On the other hand, recent developments in fiber optical transmission has made it possible to implement inexpensive transmission lines incorporating extremely-high bit rates, and the demands for transmitting various signals from many different kinds of signal sources, as well as signal processors, such as electronic computers, have become strong. In such a case, the use of time division multiplexing techniques to effectively combine a plurality of these signal channels will be required to realize a flexible and economical communications network. However, conventional time division multiplexing techniques make it necessary to convert the respective signals from the plurality of asynchronous channels into synchronous digital signals and also require use of transmission lines which are especially designed to provide clock rates for such particular application, which seems to entail many practical problems.
To eliminate the above-mentioned problem, there has been previously proposed a transmission system capable of direct time division multiplex transmission of asynchronous digital signals from a plurality of channels having arbitrary clock rates, as disclosed in U.S. application Ser. No. 118,919, filed Feb. 6, 1980, entitled "Digital Signal Communication System", now U.S. Pat. No. 4,330,856, issued May 18, 1982. In this system, the digital signals to be transmitted are converted into a composite pulse stream having a faster transmission clock rate than that of respective original signals to sequentially multiplex the original digital signals from a plurality of asynchronous channels in the time division arrangement.
Though this prior communication technology is efficient in time division multiplexing digital signals from a plurality of asynchronous channels having different bit rates, it is necessary to process and arrange the respective channel signals in the order of the clock rates of the original digital signals to be multiplexed. The communication technology on which this prior system is based also suffers from another practical problem, that is, burst signals, such as alarm signals, that appear sporadically cannot be multiplexed.